---
description: How can you enhance data discovery?
---

# Metadata

Metadata plays a **crucial role** in asset **discovery**, providing essential information such as **asset type, name, creation date, and licensing details**. Each data asset can have a [decentralized identifier (DID)](identifiers.md) that resolves to a DID document ([DDO](ddo-specification.md)) containing associated metadata. The DDO is essentially a collection of fields in a [JSON](https://www.json.org/) object. To understand working with OCEAN DIDs, you can refer to the [DID documentation](identifiers.md). For a more comprehensive understanding of metadata structure, the [DDO Specification](ddo-specification.md) documentation provides in-depth information.

<figure><img src="../.gitbook/assets/gif/data_everywhere.gif" alt=""><figcaption><p>Data discovery</p></figcaption></figure>

In general, any dApp within the Ocean ecosystem is required to store metadata for every listed dataset. The metadata is useful to determine which datasets are the most relevant.

So, for example, imagine you're searching for data on Spanish almond production in an Ocean-powered dApp. You might find a large number of datasets, making it difficult to identify the most relevant one. What can we do about it? :thinking: This is where metadata is useful! The metadata provides valuable information that helps you identify the most relevant dataset. This information can include:

* **name**, e.g. “Largueta Almond Production: 1995 to 2005”
* **dateCreated**, e.g. “2007–01–20”
* **datePublished**, e.g. “2022–11–10T12:32:15Z”
* **author**, e.g. “Spanish Almond Board”
* **license**, e.g. “SAB Data License v4”
* technical information about the **files**, such as the content type.

Other metadata might also be available. For example:

* **categories**, e.g. \[“agriculture”, “economics”]
* **tags**, e.g. \[“Europe”, “Italy”, “nuts”, “almonds”]
* **description**, e.g. “2002 Italian almond production statistics for 14 varieties and 20 regions.”
* **additionalInformation** can be used to store any other facts about the asset.

### **Overview**

DIDs and DDOs follow the [specification defined by the World Wide Web Consortium (W3C)](https://w3c-ccg.github.io/did-spec/).

[**Decentralized identifiers**](identifiers.md) (DIDs) are a type of identifier that enable verifiable, decentralized digital identity. Each DID is associated with a unique entity, and DIDs may represent humans, objects, and more. A **DID Document** (DDO) is a JSON blob that holds information about the DID. Given a DID, a _resolver_ will return the DDO of that DID.

Decentralized identifiers (DIDs) are a type of identifier that enable verifiable, decentralized digital identity. Each DID is associated with a unique entity, and DIDs may represent humans, objects, and more.

#### Rules for DID & DDO

An _asset_ in Ocean represents a downloadable file, compute service, or similar. Each asset is a _resource_ under the control of a _publisher_. The Ocean network itself does _not_ store the actual resource (e.g. files).

An _asset_ has a DID and DDO. The DDO should include metadata about the asset, and define access in at least one [service](ddo-specification.md#services). Only _owners_ or _delegated users_ can modify the DDO.

All DDOs are stored on-chain in encrypted form to be fully GDPR-compatible. A metadata cache like [_Aquarius_](aquarius/) can help in reading, decrypting, and searching through encrypted DDO data from the chain. Because the file URLs are encrypted on top of the full DDO encryption, returning unencrypted DDOs e.g. via an API is safe to do as the file URLs will still stay encrypted.

#### Publishing & Retrieving DDOs

The DDO is stored on-chain as part of the NFT contract and stored in encrypted form using the private key of the [_Provider_](provider/). To resolve it, a metadata cache like [_Aquarius_](aquarius/) must query the [Provider](provider/) to decrypt the DDO.

Here is the flow:

<figure><img src="../.gitbook/assets/architecture/publish_and_retrieve_ddos.png" alt=""><figcaption><p>DDO Flow</p></figcaption></figure>

To set up the metadata for an asset, you'll need to call the [**setMetaData**](https://github.com/oceanprotocol/contracts/blob/9e29194d910f28a4f0ef17ce6dc8a70741f63309/contracts/templates/ERC721Template.sol#L247) function at the contract level.

* [**\_metaDataState**](ddo-specification.md#state) - Each asset has a state, which is held by the NFT contract. One of the following: active (0), end-of-life (1), deprecated (2), revoked (3), ordering temporarily disabled (4), and asset unlisted (5).
* **\_metaDataDecryptorUrl** - You create the DDO and then the Provider encrypts it with its private key. Only that Provider can decrypt it.
* **\_metaDataDecryptorAddress** - The decryptor address.
* **flags** - Additional information to represent the state of the data. One of two values: 0 - plain text, 1 - compressed, 2 - encrypted. Used by Aquarius.
* **data -** The [DDO](ddo-specification.md) of the asset. You create the DDO as a JSON, send it to the [Provider](provider/) that encrypts it, and then you set it up at the contract level.
* **\_metaDataHash** - Hash of the clear data **generated before the encryption.** It is used by Provider to check the validity of the data after decryption.
* **\_metadataProofs** - Array with signatures of entities who validated data (before the encryption). Pass an empty array if you don't have any.

{% code overflow="wrap" %}
```solidity
function setMetadata(uint8 _metaDataState, string calldata _metaDataDecryptorUrl
  , string calldata _metaDataDecryptorAddress, bytes calldata flags, 
  bytes calldata data,bytes32 _metaDataHash, metaDataProof[] memory _metadataProofs) external {
  require(
      permissions[msg.sender].updateMetadata,
      "ERC721Template: NOT METADATA_ROLE"
  );
  _setMetaData(_metaDataState, _metaDataDecryptorUrl, _metaDataDecryptorAddress,flags, 
  data,_metaDataHash, _metadataProofs);
}
```
{% endcode %}

{% hint style="info" %}
While we utilize a specific DDO structure, you have the flexibility to customize it according to your unique requirements. However, to enable seamless processing, it is essential to have your own Aquarius instance that can handle your modified DDO.
{% endhint %}

{% hint style="info" %}
As developers, we understand that you eat, breathe, and live code. That's why we invite you to explore our [ocean.py](ocean.py/publish-flow.md#publishing-alternatives) and [ocean.js](ocean.js/update-metadata.md) pages, where you'll find practical examples of how to set up and update metadata for an asset :computer:
{% endhint %}

You'll have more information about the DIDs, on the [Identifiers](identifiers.md) page.